Process for preparing sand cores



United States Patent 3,431,969 PROCESS FOR PREPARING SAND CORES Janis Robins, St. Paul, Minn., assignor to Ashland Oil &

Refining Company, Ashland, Ky., a corporation of Kentucky No Drawing. Filed Aug. 15, 1966, Ser. No. 572,200 U.S. Cl. 16443 5 Claims Int. Cl. B22c 1/22; C07c 55/06 ABSTRACT OF THE DISCLOSURE The bench life of foundry core binders containing polyisocyanates and suitable hydroxy-containing co-reactants (e.g., oil modified alkyd resins) is extended by addition of a free acid (e.g., oxalic acid) to the foundry mix.

This invention relates to core binders having extended bench life. In another aspect, this invention relates to a foundry process for making sand cores using organic core binders containing an isocyanate which have an extended bench life.

In the foundry art, cores and molds for use in making metal castings are usually prepared from mixtures of sand with a binding amount of a polymerizable or curable binder. Such mixtures are referred to herein as foundry mixes. The amount of binder used typically is from 0.5 to 3% based on the weight of the sand. Frequently, minor amounts of other materials are also included in foundry mixes, e.g., iron oxide, ground flax fibers, and the like. The binder permits a foundry mix to be molded or shaped into the desired fonm, usually in a pattern box or mold, and thereafter cured to form a self-supporting structure (i.e., a sand core or mold).

In recent years, the foundry art has been provided with core binders containing polyisocyanates and suitable co-reactants. See, for example, US. Patent No. 3,255,500 issued to I. J. Engel, R. J. Sch-afer and V. L. Guyer on June 14, 1966. Certain binders of this general type (i.e., containing an isocyanate), have a number of outstanding advantages. One such advantage is their ability to effectively and rapidly form cured cores at room temperature without the use of gaseous catalysts. Optionally, cores prepared with such binders can be baked to accelerate their cure. The ability of these core binders to effectively and rapidly form cured cores at room temperature is a disadvantage in some situations, i.e., the bench life of the core binder is too short for some uses. As used in this application, the term bench life refers to the time interval existing between the time the foundry mix is prepared and the time when the mix can no longer be readily and effectively introduced in a pattern. This time interval is conveniently measured by subjecting a cylindricallyshaped sample of the sand mix to compressive force and measuring the time required for the sample to cure to the point at which one pound per square inch of compressive force is necessary to cause it to collapse. It is often referred to as the loss of workability. Also, as used in this application, the term strip time refers to the time interval existing between the time the prepared foundry mix is placed in a pattern or mold and the time at which an acceptable level of stripping strength is reached, i.e., time required for foundry mix to cure sufficiently so that the pattern or mold can be stripped away. This time interval can be measured similarly to the measurement of bench life, except that the level of compressive strength required is 20 pounds per square inch.

-It has now been discovered that the bench life of core binders containing polyisocyanates and suitable hydroxycontaining co-reactants can be substantially extended by the simple addition of a free acid (e.g., oxalic acid) to the foundry mix. For example, one popular commercially available isocyanate binder composition contains both an aromatic polyisocyanate and an oil-modified alkyd resin. When this binder composition was used in a manner recommended by the manufacturer and a small amount of a free acid was simply added to the foundry mix, the foundry mix had a bench life of about 25% or more longer than the bench life of the untreated foundry mix.

The acids which can be incorporated into the foundry sand mixes, comprising sand and an isocyanate core binder, to extend the bench life thereof have the formula:

wherein R is hydrogen or an organic radical (either monovalent or divalent) which is unsubstituted or substituted With a non-interfering substituent, and x is a positive integer from 1 to 2. R is preferably hydrogen or a saturated, divalent organic radical and x is preferably 2.

Although a wide variety of acids can be used, including aliphatic monobasic acids, such as acetic, propionic, butyric and the like, aromatic monobasic acids, such as benzoic, toluylic, and the like, dibasic acids from the oxalic acid series, such as, oxalic acid, malonic acid and the like, aromatic dicarboxylic acids, such as isophthalic are especially preferred.

The amount of acid included in the foundry mix can vary widely. Most often, the range of useable amounts will be from an effective amount up to about 50% based on the weight of the isocyanate used in the binder. More preferably, the acid will be present in an amount from 1.0 to 25 based on the weight of the isocyanate.

Since the efiect of certain acids on the bench life vary widely, another method of determining the amount of acid which should be used is that amount suflicient to extend the bench life 25 longer than that of a mix containing no added acid.

The binder compositions which can be benefited by the practice of this invention are known tothe art and are those which contain a polyisocyanate, usually, with hydroxyl-containing material as a co-reactant. Such isocyanate-hydroxyl binder systems are co-reacted at or about the time of use in the presence of sand. Typically, the reactive ingredients of such binder compositions are sold, shipped and stored in separate packages (i.e., a multiple package core binder) to avoid undesirable deterioration due to premature reaction between the components. Solvents, catalysts, various additives and other known binders can optionally be used in conjuction with these essential ingredients, i.e., used with the polyisocyanate and its co-reactant.

Typically, cyclic and acylic polyisocyanates containing from 2 to 5 isocyanate groups are employed. If desired, mixtures of polyisocyanates can be employed. Less preferably, isocyanate prepolymers formed by reacting excess polyisocyanates with a polyhydric alcohol (e.g., a prepolymer of toluene diisocyanate and ethylene glycol) can be employed. Suitable polyisocyanates include the aliphatic polyisocyanates such as hexamethylene diisocyanate; alicyclic polyisocyanates (such as 4,4-dicyclohexylmethane diisocyanate; and aromatic polyisocyanates 2,4- and 2,6-toluene diisocyanate, and diphenyl methane diisocyanate and the dimethyl derivatives thereof). Further examples of suitable polyisocyanates are 1,4-naphthalene diisocyanate; triphenyl methane triisocyanate; xylylene diisocyanate and methyl derivatives thereof; polymethylene polyphenyl isocyanate, chlorophenylene 2-, 4-diisocyanate, and the like.

All polyisocyanates do not serve with the same effectiveness. While the aforementioned polyisocyanates are, to a greater or lesser degree, effective in practicing the present invention, there are significant advantage associated with the use of cyclic polyisocyanates, especially the aromatic polyisocyanates, as constrasted to the aliphatic polyisocyanates. In general, aromatic polyisocyanates impart more rigidity to cores than do the aliphatic polyisocyanates. Poly-methylene polyphenyl isocyanate, diphenylmethane diisocyanate, triphenylmethane triisocyanate, and mixtures thereof are preferred because of their high degree of reactivity, their desirable coreforming properties, and their low vapor pressure. The latter minimizes any possible toxicity problems.

The coreactant for the polyisocyanate can be a hydroxyl-containing material, a natural drying oil, an airhardenable petroleum polymer, a synthetic drying oil such as dicyclopentadiene copolymer or a butadiene/ styrene copolymer, or the like. Mixtures of such co-reactants can be used. Co-reactants containing free or reactant hydroxyl groups are preferred.

The hydroxyl-containing material to be co-reacted with the isocyanate can be a polyhydric alcohol (e.g., pentaerythritol). However, high molecular weight bydroxyl-containing materials are preferred. Preferably, the hydroxyl-containing co-reactant will be an alkyd resin (e.g., an oil-modified oil resin), a hydroxyl-terminated polyester (e.g., the alcoholysis products of fatty trigylcerides) and polymers and copolymers containing reactive hydroxyl groups (e.g., hydroxy-alkyl acrylate copolymers). Hydroxyl-containing materials which can be hardened, at least in part, by air oxidation are particularly preferred. Thus, drying oil-modified alkyd resins, as well as various unsaturated fatty acid esters and unsaturated fatty acid modified polyesters are preferred. Oil-modified alkyd resins are the most preferred hydroxylcontaining reactants.

It is preferred to employ oil-modified alkyd resins which have been prepared by co-reacting the following three classes of ingredients:

(A) Polyhydric alcohols having at least three hydroxyl groups, e.g., glycerol, pentaerythritol, trimethylol propane and the like. Pentaerythritol is preferred. Mixtures of polyhydric alcohols can be used. While glycols can be used, better results are obtained if such glycols are used in conjunction with the polyhydric alcohols (using a mixture of ethylene glycol and pentaerythritol). Ordinarily, only the polyhydric alcohols will be used.

(B) Polycarboxylic acids (or their anhydrides) such as maleic acid, fumaric acid, phthalic acid, pht-halic anhydride, isophthalic acid, chlorendic acid, and the like. The various phthalic acids (particularly isophthalic acid and phthalic anhydride) are preferred. Mixtures of acids can also be used.

(C) Oil, such as soybean oil, linseed oil, cotton seed oil, castor and dehydrated castor oils, tall oil, tung oil, fish oil and the like. Mixtures of oils can be used. Linseed oil is preferred.

The more preferred oil-modified alkyd resins will contain at least 40 weight percent oil (based on the total Weight of the alkyd resin formulation). More desirably, the alkyd resins will contain at least 50 weight percent oil on the same basis (i.e., a long oil alkyd). It should be pointed out that oil-modified alkyd resins can also be prepared (as is known in the resin art) from fatty acids rather than the corresponding oils or glycerides. With alkyd resins, especially the oil-modified alkyd resins, the hydroxyl 'value should be at least 25 and preferably above 50. The upper limit of hydroxyl value is only limited by practical considerations, e.g., viscosity. For most ordinary applications, oil-modified alkyd resins having hydroxyl values of from 50 to 250, e.g., 60 to 150 are desirable.

Catalysts are used in conjunction with isocyanate core binders. The catalysts which are usually employed are those which accelerate the air oxidation or hardening of the oil-modified alkyd resin, those which accelerate reaction between the polyisocyanate and the oil-modified alkyd resin, and those which do both, The amount of catalyst employed will be a catalytic amount, with the total amount of catalyst(s) usually ranging from .01 to 15%, based on the combined weight of the polyisocyanate and the oil-modified alkyd resin. More frequently, from 0.1 to 10%, e.g., 0.25 to 5% catalyst will be used, on the same basis. The choice of catalyst and the amount thereof will affect the curing rate of the system. Metal naphthenates (e.g., cobalt naphthenate and lead naphthenate) are effective catalysts for both the isocyanate/hydroxyl reaction and the air oxidation of the hydroxyl-containing alkyd resin, the latter being their primary function. Sodium perborate is also useful in promoting the oxygen cross-linking of the drying oil, although its use is not particularly preferred. Dibutyl tin dilaurate is an effective catalyst for promoting the urethane reaction. We offer these catalysts as exemplary of those which can be employed in this invention. However, other known driers can be used to promote the air-drying feature of the present invention as well as the curing of the isocyanate portion. We prefer to use cobalt naphthenate in combination with other known catalysts, such as lead naphthenate and dibutyl tin dilaurate in the practice of this invention since it appears that the presence of cobalt naphthenate enables the foundry worker to better control the bench life of the core binder-sand mix by the method of this invention.

The isocyanate binder system of the present invention can optionally be used in combination with other known binder systems. Also, the foundry mixes of this invention can optionally include other ingredients, such as, iron oxide, ground flax fibers, wood cereal, refractory flours, pitch, etc.

When combining the isocyanate and its co-reactant (e.g., an oil-modified alkyd resin) with sand, at or about the time a sand core is to be made, it is common to use from 5 to 150 parts by weight of polyisocyanate per 100 parts by weight of the co-reactant. More frequently, from 5 to parts, e.g., from 8 to 50 parts of polyisocyanate will be used on the same basis. With our preferred embodiment, it is common to use from 10 to 40 parts, e.g., 15 to 30 parts by weight of polyisocyanate on the same basis.

The total amount of isocyanate binder employed (i.e., the total weight of isocyanate plus co-reactant) based on the weight of sand, will be a binding amount of up to 10%. Generally, the amount of binder (on the same basis) will be from 0.5 to 5 weight percent, e.g., 1 to 3 weight percent. In mixing the acid, isocyanate, co-reactant and catalyst with sand, it is advantageous to first mix the co-reactant (e.g., or a modified alkyd resin or polyester) with the sand (and other optional ingredients), then add the catalyst with mixing, and finally add the polyisocyanate and the acid. The catalyst can be added to the polyisocyanate and acid mixture prior to the addition of the mixture to the sand and eo-reactant mixture. The resulting foundry mix will typically remain workable or plastic at room temperature for from 25 to 60 minutes.

The mixture is then molded or shaped into desired form, usually in a pattern box or mold, and thereafter cured to form a sand core. Depending upon the nature of the polyisocyanate, co-reactant, and catalyst, the curing will be accomplished by simply allowing the binder to react at room temperature or by baking or a combination of both techniques.

The present invention will be further understood by reference to the following specific examples which include the :best mode known to the inventors for practicing their invention. Unless otherwise indicated, all parts and percentages are by weight.

Example I As a basis for comparison, a foundry sand mix was prepared using a commercially available isocyanate core binder (i.e., Lino-Cure, a product of the Archer-Daniels- Midland Company). This core binder consisted of three parts or packages. The first part wa a mixture of 47 parts of an oil-modified alkyd resin, 29 parts of a petroleum polymer and 24 parts of solvent (mineral spirits).

The oil-modified alkyd resin was prepared by reacting approximately 64 parts of linseed oil, parts of pentaerythritol, and 21 parts of isophthalic acid. The hydroxyl number of the alkyd resin was approximately 105. The second part or package (i.e., the catalyst) contained a mixture of cobalt neodecanoate, lead neodecanoate and dibutyl tin dilaurate in a weight ratio of 8:1: 1. The third package contained aromatic polyisocyanate (diphenylmethane diisocyanate).

The foundry sand mix was prepared by mixing 10,000 parts Wedron silica sand, 150 parts of package A (i.e., the oil-modified alkyd), 9 parts of package B (i.e., the catalyst) and 30 parts of package C (i.e., the isocyanate).

The foundry mix was found to have a bench life of 21 minutes and a strip time of 36 minutes.

Example II The procedure of Example I was followed exactly to prepare two foundry sand mixes containing sand and parts A, B and C as described in Example I. In addition, however, 0.75 part of acetic acid were added to one mix and 6.5 parts of acetic acid were added to the other.

More specifically, two foundry sand mixes were prepared by mixing in each of two containers, 10,000 parts Wedron silica sand, 150 parts oil-modified alkyd resin, 9 parts catalyst and 30 parts isocyanate as in Example I. To the mix in container 1 were added 0.75 part glacial acetic acid, and to the mix in container 2 were added 6.5 parts glacial acetic acid.

The first mix was found to have a bench life of 25 minutes and a strip time of 46 minutes; while the second mix had a bench life of 31 minutes and a strip time of 65 minutes. The bench life and strip time were measured exactly as in Example I.

Example III The procedures of Examples I and II were followed to prepare two foundry sand mixes containing sand and parts A, B and C as disclosed in Example I in addition to small amounts of benzoic acid.

More specifically, the two mixes were prepared by mixing, in one container, 10,000 parts of Wedron silica sand, 150 parts alkyd resin, 9 parts catalyst, parts isocyanate and 0.75 part benzoic acid. In another container were mixed 10,000 parts sand, 150 parts resin, 9 parts catalyst, 30 parts isocyanate and 6.5 parts benzoic acid.

The first mix was found to have a bench life of 24 minutes and a strip time of 45 minutes, While the second mix had a bench life of 29 minutes and a strip time of 54 minutes. The bench life and strip time were measured exactly as in Examples I and II.

Example IV The procedure of Examples I, II and III were followed to prepare two foundry sand mixes containing sand and parts A, B and C as disclosed in Example I, in addition to small amounts of oxalic acid.

More specifically, two mixes were prepared by mixing, in one container, 10,000 parts Wedron silica sand, 150 parts alkyd resin, 9 parts catalyst, 30 parts isocyanate and 0.75 part oxalic acid. In another container were mixed 10,000 parts Wedron silica sand, parts alkyd resin, 9 parts catalyst, 30 parts isocyanate and 6.5 parts oxalic acid. 1

The first mix had a bench life of 35 minutes and a strip time of 69 minutes while the second mix had a bench life of 55 minutes and a strip time of 112 minutes.

The foregoing examples have shown that the bench life of isocyanate core binders can be substantially increased by the addition, to the foundry sand mix, of an acid. The examples further showed that dibasic acids, such as oxalic acid, extend the bench life of isocyanate core binders to an even greater extent than do monobasic aliphatic acids or aromatic acids.

Although the present invention has been described with a certain degree of particularity, it will be realized that numerous minor changes and variations, falling within the spirit and scope of this invention, will become obvious to those skilled in the art. It is not intended that this invention be limited to any of the materials which have been specifically mentioned for the sake of illustration, nor by the specific portions which have been given for the sake of illustration.

What is claimed is:

1. In a process for preparing sand cores wherein an organic binder containing an isocyanate and a suitable coreactant for said isocyanate is mixed with sand to form a foundry mix, the foundry mix is shaped and thereafter cured to form a sand core by reacting said isocyanate and said co-reactant in the presence of said sand and catalyst, the improvement which comprises including, in said foundry mix, a carboxylic acid in an amount sufficient to extend the bench life of the said binder.

2. The process of claim 1 wherein the carboxylic acid has the formula: R(COOH) wherein R is a hydrogen or hydrocarbon radical containing from 1 to 20 carbon atoms and x is 1 or 2.

3. The process of claim 1 wherein said co-reactant comprises an oiZ-modified alkyd resin having a hydroxyl value of at least 25 and said isocyanate comprises aromatic isocyanate having 2-5 isocyanate groups.

4. The process of claim 3 wherein said isocyanate is selected from the group consisting of polymethylene polyphenyl isocyanate, diphenylmethane diisocyanate, and triphenyl methane triisocyanate; said alkyd resin is prepared from linseed oil, isophthalic acid and pentaerythritol and said curing is accomplished at room temperature.

5. The process of claim 2 wherein the acid is a dibasic acid of the oxalic acid series.

References Cited UNITED STATES PATENTS 9/1964 Archer et al. l6443 X 6/1966 Engel et al. 16443 US. Cl. X.R. 

